Method of producing magnetic alloys and novel product

ABSTRACT

Described herein is a method of processing an alloy of iron, cobalt, and at least one of vanadium, chromium, molybdenum, and tungsten to develop a novel product having an improved combination of magnetic properties and strength. The method involves continuously annealing the alloy so as to produce an average grain size of ASTM -12 or finer.

United States Patent Thomas B. MeCm 1mm lend. Pa.

Appl- No. 829.783

Filed June 2, 1969 Patented Nov. 23, 1971 Assignee Allegheny Ludlum Steel Corporation Pittsburgh, Pa.

Inventor METHOD OF PRODUCING MAGNETIC ALLOYS AND NOVEL PRODUCT 4 Claims, No Drawings US. Cl 148/122, 75/123, 75/170,148/31.55,148/121 1nt.Cl H011 1/14, H01f1/16,C22c 19/00 Field 01 Search 148/ 120,

[56] References Cited UNITED STATES PATENTS 1,862,559 6/1932 White et a1. 1481121 X 2,512,358 6/1950 McGeary 148/120 X 2,717,223 9/1955 Binstock et a1.. 148/122 3,024,141 6/1962 Burket et a1. 148/120 X 3,065,118 11/1962 Wawrousek et a1. 148/3 1.55 X

Primary Examiner-4.. Dewayne Rutledge Assistant Examiner-G. K. White Attorneys-Richard A. Speer and Vincent G. Gioia METHOD OF PRODUCING MAGNETIC ALLOYS AND NOVEL PRODUCT This invention relates to a method of processing an alloy of iron, cobalt, and at least one metal of the group consisting of vanadium, chromium, molybdenum, and tungsten, which results in a superior combination of magnetic properties and yield strength. More particularly, the invention relates to a method of manufacturing and to an annealed product which possesses a DC magnetic induction of at least 22,000 gausses at 100 oersteds and a yield strength of at least 70,000 p.s.i.

A class of alloys containing iron, cobalt, and vanadium are known to be useful for magnetic applications because they possess superior magnetic properties at high inductions. A typical composition of this class is an alloy nominally containing 49 percent iron, 49 percent cobalt, and 2 percent vanadium which is known as Vanadium Permendur. The vanadium is included in these iron-cobalt alloys to render the composition workable. Other elements such as chromium, molybdenum, and tungsten may, however, be substituted for vanadium. The magnetic properties of the iron and cobalt-containing alloys are developed upon recrystallization of as-cold-rolled material. For these alloys, recrystallization occurs at about l,270 F.

As the annealing temperature is increased, the magnetic properties improve, however, the grain size increases considerably and the yield strength decreases appreciably.

The conventional manufacture of Vanadium Permendur articles typically involves punching laminations from as-coldrolled strip and box annealing the punchings in a temperature range of about l,300-1,600 F. The temperature limits are important because higher annealing temperatures result in poor magnetic properties due to the formation of a nonmagnetic phase. A problem of the conventional practice is that box annealing requires a relatively lengthy residence time at temperature to assure substantial unifonn heating throughout the material. An additional disadvantage of box annealing is the difficulty in controlling the temperature within the narrow range necessary to achieve a superior combination of yield strength and acceptable magnetic properties. When producing punched laminations by box annealing the laminations are stacked, therefore heating to temperature must be done slowly to avoid overheating and extensive time is required to insure that all laminations have reached temperature. This longer time at temperature, i.e., residence time is responsible for grain growth. As a result of the foregoing, box annealed ironcobalt alloy products of the type described are produced which, though possessing superior magnetic properties, have relatively inferior mechanical properties. At least partly as a consequence of the long residence time employed in box annealing, the average grain size of material produced is generally of ASTM No 7 to ASTM No. 9. It has now been discovered that iron and cobalt-containing alloys of the type described may be processed to provide a high level of magnetic properties with considerably improved yield strength. In accordance with the invention an alloy of iron, cobalt, and at least one of the group consisting of vanadium, chromium, molybdenum, and tungsten is continuously annealed in a nonoxidizing atmosphere at a temperature of about l,300-l ,600 F. to produce a product having an average grain size of ASTM No. 12 or finer and then cooled at a rate which precludes grain growth beyond this limit. The resulting product is in the annealed condition and has a DC magnetic induction of at least 22,000 gausses at 100 oersteds with a yield strength of at least 70,000 p.s.i. This level of magnetic properties compares well with box annealed Vanadium Permendur but the yield strength is much better.

For the purposes of producing punched laminations from as-cold-rolled strip by continuously annealing in accordance with the invention, any suitable furnace may be used. One such is a conventional belt type furnace wherein the furnace temperature and speed of the belt may be controlled. The laminations can be continuously conveyed into, through, and out of the furnace on the belt. Of course, it is not necessary to continuously anneal punched articles. It is possible, in accordance with the invention, to first continuously anneal the strip and then to produce laminations or other components from the annealed stn'p.

Regardless which type of continuous annealing furnace is used, it is advantageous to employ a temperature and line speed such that the alloy is just recrystallized or partially recrystallized, however, the fineness of the grain size is the controlling criteria. In practicing the invention it is necessary to assure a combination of line speed and temperature so that a grain size of ASTM No. 12 or finer is achieved. This grain size is necessary to assure a high yield strength of at least 70,000 p.s.i., but some recrystallization is also required in order to develop the satisfactory magnetic properties typical of these alloys. Naturally the particular temperature and line speed used to obtain the desired fine grain structure depends upon the characteristics of the furnace used, strip thickness, width, etc.

It may be advantageous to avoid full recrystallization because such material may possess a larger grain size than ASTM No. 12 and consequently will not have the high yield strength characteristic of the product produced in accordance with the invention.

The following examples will serve to illustrate the practice of one presently preferred embodiment of the invention. A series of ring samples of 2.5 inches o.d. X 1.5 inches i.d. were prepared from 0.014 inch thick as-cold-rolled strips of two heats of Vanadium Permendur having the composition described in table I. One set of samples was continuously annealed in a belt type furnace having a hot zone of about 5 feet. The furnace temperature was set at l,385 F. and a belt speed of 6 inches per minute was used. A nonoxidizing atmosphere comprising hydrogen having a dew point of F. or dryer was introduced to the furnace. Because of leakage, it was determined that the dew point of the hydrogen in the furnace was about l0 F. Another series of samples were continuously annealed in a catenary type furnace having a hot zone of 13 feet. The samples were attached to a strip of stainless steel by drilling holes in the stainless steel strip and wiring the samples to the strip. The furnace operation was controlled to provide a line speed such as to provide 2 minutes at a temperature of l,385 F. The incoming atmosphere was dry hydrogen of 80 F. dew point or dryer but the dew point of the gas in the furnace was about 0 F.

For comparison another series of samples were prepared and given a box anneal at l,385 F. for 4 hours at temperature to simulate present commercial practice. Again hydrogen atmosphere of -80 F. dew point or dryer was used.

The results of tensile tests are described in table ll and the magnetic properties of the samples are shown in table Ill.

TABLE I Heat No. 0 Mn Si V Co Fe 7-2629 .004 .026 .024 1. 93 49.19 Balance. 7-3205 .013 040 .024 2.02 48. Bal.

TABLE II Elon 0.2% Y.S. T.S. in 2 Armeal conditions Heat N o. (p.s.i.) (p.s.i.) percent Btilgstfiygpg iuntiiafie 1set at an e t speed of 6/mtnute 7-2629 76: 000 123: 600 1010 H205 90, 900 153, 200 9. 0 2t' %fi se a an ne speed ot6/minute 72629 $213 {19% 5-2 771600 129: 12s 10: 0 74205 76, 460 120,980 9. 0

Box anneal, 1,385 F., 4

hours 7-2629 2 238 210 5'8 59, 410 108, 490 8. 5 H205 56, 440 96, 440 6. a

IOIOIIIO It is apparent from the foregoing that the product produced by continuously annealing in accordance with the invention possesses a 0.2 percent yield strength of 70,000 p.s.i. or higher. Moreover. there is no loss in ductility as measured by elongation in the tensile test. It is also evident that the magnetlc properties although somewhat lower as compared to the box annealed material, are quite satisfactory at high inductions and compare very well with the box annealed Vanadium Permendur.

I claim:

1. A method of processing an alloy having nominally 49 percent iron, 49 percent cobalt and 2 percent vanadium, to develop an improved combination of magnetic properties and strength which comprises continuously annealing said alloy in a nonoxidizing atmosphere at a temperature from about 1.300 F. to 1,600 F. for a time sufficient to produce an average grain size of ASTM No. 12 or finer and cooling at a rate to preclude grain growth beyond said limit to produce an annealed product having a DC magnetic induction of at least 22,000 gausses at oersteds and a yield strength of at least 70,000 p.s.i.

2. A method according to claim I wherein said nonoxidizing atmosphere comprises dry hydrogen.

3. An annealed alloy having nominally 49 percent iron, 49 percent cobalt and 2 percent vanadium, an average grain size of not coarser than ASTM No. 12, a DC magnetic induction of at least 22,000 gausses at 100 oersteds and a yield strength of at least 70,000 p.s.i.

4. An alloy according to claim 3 having a partially recrystallized microstructure.

i t t i 

2. A method according to claim 1 wherein said nonoxidizing atmosphere comprises dry hydrogen.
 3. An annealed alloy having nominally 49 percent iron, 49 percent cobalt and 2 percent vanadium, an average grain size of not coarser than ASTM No. 12, a DC magnetic induction of at least 22,000 gausses at 100 oersteds and a yield strength of at least 70,000 p.s.i.
 4. An alloy according to claim 3 having a partially recrystallized microstructure. 